[libc] Bring libc in-tree

Moving libc from its separate repo into this one, minor resulting build
fixes, and a hacky way to add -I for libc headers in builds.

src/libraries/libc/j6libc/closeall.c

@@ -0,0 +1,21 @@
+/* _PDCLIB_closeall( void )
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include <stdio.h>
+
+extern struct _PDCLIB_file_t * _PDCLIB_filelist;
+
+void _PDCLIB_closeall( void )
+{
+    struct _PDCLIB_file_t * stream = _PDCLIB_filelist;
+    struct _PDCLIB_file_t * next;
+    while ( stream != NULL )
+    {
+        next = stream->next;
+        fclose( stream );
+        stream = next;
+    }
+}